Light emitting diodes are widely used as indicator lamps in association with various electric components and circuits since light emitting diodes consume relatively little power and generate little heat as compared with conventional incandescent indicator lamps. One such application is found in indicator lamps for indicating the states of limit switches. However, light emitting diodes are sensitive to voltage fluctuations. If the applied voltage is excessively high, the light emitting diode may be destroyed. Conversely, if the applied voltage is excessively low, the brightness of the light emitting diode may become insufficient and may even stop producing any light. Therefore, there has been a demand for light emitting diode indicators which can perform satisfactorily even when there is a fluctuation in the voltage applied thereto.
Light emitting diode indicators may be adapted to different voltages by properly selecting the values of the resistors connected in series with the corresponding light emitting diodes. Therefore, it has been necessary to stock a large number of light emitting diode indicators having different voltage ratings in order to meet different requirements. This means an increase in the manufacturing cost, and more significantly an increase in the maintenance cost of each system using light emitting diode indicators.